Load driving device, and lighting apparatus and liquid crystal display device using the same

ABSTRACT

A light emitting load driving device includes a plurality of constant current sources structured to be serially connected to a plurality of light emitting loads connected in parallel respectively, and structured to control a current flowing through the plurality of light emitting loads connected in parallel; a plurality of load connection terminals structured to be connected to the plurality of light emitting loads connected in parallel and the plurality of constant current sources respectively; a control circuit structured to be controlled based on a plurality of terminal voltage applied to the plurality of load connection terminals and a reference voltage, and structured to control a voltage output portion generating an output voltage provided to the plurality of light emitting loads connected in parallel so that both of a lowest terminal voltage applied to the plurality of load connection terminals and the reference voltage are equalized with respect to each other.

TECHNICAL FIELD

This disclosure relates to a device for driving a load (load drivingdevice) with the output voltage by converting an inputted power supplyvoltage by a dc-dc conversion type power source circuit, and to aportable apparatus equipped with such a load driving device.

BACKGROUND

There have been many devices in use for driving loads such as LEDs,utilizing a dc-dc conversion type power supply circuit adapted toprovide an output voltage different from an inputted power supplyvoltage. A typical load driving device has a power supply circuit thatgenerates a predetermined output voltage and an output current fordriving a load, as disclosed in Japanese Patent Application Laid OpenNo. 2001-313423. For this purpose, the level of the output voltage orthe output current supplied to the load is measured to establish adetection voltage or detection current, which is fed back to a controlcircuit of the power supply circuit.

In such conventional load driving device, the detection voltage isobtained by dividing the output voltage in a voltage dividing circuithaving a high resistance. The detection current is obtained by detectingthe potential drop across a resistor (referred to as voltage detectionresistor) connected in series with the load, whereby the load currentflows through the resistor. The detection voltage (or detection current)is compared to a reference value so that the output voltage (current)outputted from the power supply circuit is controlled based on thecomparison.

In a portable electronic device such as a cellular phone, the loadcurrent is sometimes increased or decreased within a permitted range inresponse to a request made during service. For example, when the load isa light emitting diode (LED), a request is made to regulate theluminance of the LED to an arbitrary level.

In such an example, the voltage detection resistor connected in serieswith the load will increase energy loss when the load current isincreased. Therefore, the overall efficiency of the electronic devicethat includes a power supply circuit and a load disadvantageously dropswhen the load current becomes large (i.e., during a heavy duty).

In another example, a request is made to drive one load with a constantcurrent and at the same time to drive another load with a voltage abovea predetermined voltage. In such an example, conventionally it isnecessary to provide a further appropriate power supply circuit to meetindividual use conditions, which requires additional space and cost forthe power supply circuit and load.

It could, therefore, be helpful to provide a load driving device havinga dc-dc conversion type power supply circuit that generates an outputvoltage by converting a power supply voltage (an input voltage), thepower supply circuit capable of adjusting the magnitude of the loadcurrent within a predetermined range while avoiding the energy losscaused by an increase in the load current, thereby enabling efficientdriving of the load.

It could also be helpful to provide a portable apparatus equipped withsuch a load driving device.

It could further be helpful to provide a load driving device having adc-dc conversion type power supply circuit that generates an outputvoltage by converting a power supply voltage, the power supply circuitcapable of driving a multiplicity of loads having different useconditions, including at least one constant-current type load andanother type of load, and capable of adjusting the magnitude of the loadcurrent supplied to the constant-current type load within apredetermined range while maintaining the output voltage to another typeof load above a predetermined voltage.

It could still further be helpful to provide a portable electronicapparatus equipped with such a load driving device.

SUMMARY

We thus provide:

A load driving device having a power supply circuit that supplies to aload an output voltage by converting an input voltage and aconstant-current source connected in series with the load and capable ofproviding a constant current that can be adjusted in magnitude (suchcurrent hereinafter referred to as adjustable constant current andcurrent source referred to as variable-current type current source),wherein the power supply circuit is adapted to control the outputvoltage to keep constant the voltage at the node of the load andconstant-current source. The constant-current source has a currentmirror circuit constituted of a constant-current circuit providing anadjustable constant current (adjustable-current type constant-currentcircuit), an input-side current mirroring transistor connected in serieswith the constant-current circuit, and an output-side current mirroringtransistor receiving the same control input as the input-sidetransistor, wherein the adjustable constant current is supplied to theoutput-side transistor. The constant voltage is higher than thesaturation voltage of the output-side current mirroring transistor.

In addition to a dc-dc conversion type power supply circuit employed toprovide an output voltage by converting an input voltage, aconstant-current source providing an adjustable constant current mayconnect in series with a load having an operating point depending on themagnitude of the current flowing through it (e.g., a set of LEDs). Thus,it is possible to provide the load with a required magnitude of currentin a stable manner.

The output voltage of the dc-dc conversion type power supply circuit iscontrolled such that the voltage drop across the constant-current sourcebecomes equal to a reference voltage where the reference voltage is setto secure stable operation of the constant-current source. Thus, theoutput voltage of the dc-dc conversion type power supply circuit isautomatically adjusted so that a magnitude of current required byrespective LEDs for proper luminance will flow through it even when theLEDs fluctuate in luminescence characteristic.

The voltage drop across a constant-current source is controlledautomatically to become equal to the reference voltage so that thecurrent is maintained at the preset magnitude. Therefore, even if thecurrents flowing through the LEDs grow larger, there will be no suchenergy loss as would be incurred by a voltage detecting resistor. Thus,substantially no extra energy loss is caused by an increase in the loadcurrent that the load driving device can efficiently drive a load over awide range of load current.

An adjustable-current type constant-current source can be provided foreach load consisting of a set of LEDs such that the dc-dc conversiontype power supply circuit is controlled based on the lowest one of thevoltage drops across the constant-current sources. This ensures stablesupply of a predetermined constant current to each of the LEDsconstituting the load.

My load driving device comprises:

-   -   a power supply circuit for supplying to a load an output voltage        by converting an input voltage; and    -   a variable-resistance means having a resistance that varies in        response to a control signal and a current detection means for        detecting the magnitude of the current flowing through the        variable-resistance means, both means connected in series with        the load, wherein    -   the power supply circuit is fed with a first reference voltage        and a first detection voltage provided by the current detection        means, and controls the output voltage to equalize the first        detection voltage to the first reference voltage.

The variable-resistance means has a low resistance when a voltageindicative of the output voltage exceeds a predetermined voltage, andhas a resistance that increases in accord with the decrease in thevoltage indicative of the output voltage below the predeterminedvoltage.

A multiplicity of load has different load characteristics. For example,a constant-current load can be driven by a constant current with itsmagnitude varied within a predetermined range by use of a power supplycircuit such as a dc-dc conversion type power supply circuit generatingan output voltage by converting an input power supply voltage, and atthe same time another load other than constant-current type can bedriven by keeping the output voltage above a predetermined level for theload.

For a constant-current load such as a set of LEDs having an operatingpoint that depends on the magnitude of the current flowing through it,an adjustable-current type constant-current source may be connected inseries with the load. It is thus possible to provide the load with arequired magnitude of current in a stable manner.

When the output voltage exceeds the predetermined voltage, the outputvoltage of the dc-dc conversion type power supply circuit is controlledto equalize the voltage drop across the constant-current source to areference voltage, where the reference voltage is set to secure stableoperation of the constant-current source. Thus, the output voltage ofthe power supply circuit is automatically adjusted so that the magnitudeof current necessary for the set of LEDs of the load to emit apredetermined amount of light will flow through them even if the LEDs inthe load fluctuate in luminescent characteristic.

Moreover, when the output voltage tends to drop below the predeterminedvoltage due to the adjustment of current for the LEDs, the outputvoltage may be controlled to remain at the predetermined voltage. Thus,it is possible to secure the predetermined output voltage for thenon-constant-current type load.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of a load driving device inaccordance with a first example.

FIG. 2 is a graph showing the current-voltage characteristic of an LED.

FIG. 3 is a schematic circuit diagram of a constant-current source I1.

FIG. 4 is a graph showing the drive current-output voltagecharacteristic of a first dc-dc-conversion type power supply circuit ofFIG. 1.

FIG. 5 is a schematic circuit diagram of a load driving device inaccordance with a second example.

FIG. 6 is a schematic circuit diagram of a load driving device inaccordance with a third example.

FIG. 7 is a graph showing the drive current-output voltagecharacteristic of a third dc-dc-conversion type power supply circuit ofFIG. 6.

FIG. 8 is a schematic circuit diagram of a load driving device inaccordance with a fourth example.

FIG. 9 is a schematic circuit diagram of a load driving device inaccordance with a fifth example.

FIG. 10 is a schematic circuit diagram of a load driving device inaccordance with a sixth example.

FIG. 11 is a schematic circuit diagram of a load driving device inaccordance with a seventh example.

FIG. 12 is a schematic circuit diagram of a load driving device inaccordance with an eighth example.

FIG. 13 is a schematic circuit diagram of a load driving device inaccordance with a ninth example.

FIG. 14 is a schematic circuit diagram of a load driving device inaccordance with a tenth example.

FIG. 15 is a schematic circuit diagram of a load driving device inaccordance with an eleventh example.

FIG. 16 is a schematic circuit diagram of a load driving device inaccordance with a twelfth example.

FIG. 17 is a schematic circuit diagram of a load driving device inaccordance with a thirteenth example.

FIG. 18 is a schematic circuit diagram of a load driving device inaccordance with a fourteenth example.

FIG. 19 is a schematic circuit diagram of a load driving device inaccordance with a fifteenth example.

FIG. 20 is a schematic circuit diagram of a load driving device inaccordance with a sixteenth example.

FIG. 21 is a schematic circuit diagram of a load driving device inaccordance with a seventeenth example.

FIG. 22 is a schematic circuit diagram of a load driving device inaccordance with an eighteenth example.

FIG. 23 is a schematic circuit diagram of a load driving device inaccordance with a nineteenth example.

FIG. 24 is a schematic circuit diagram of a load driving device inaccordance with a twentieth example.

FIG. 25 is a schematic circuit diagram of a load driving device inaccordance with a twenty-first example.

FIG. 26 is a schematic circuit diagram of a load driving device inaccordance with a twenty-second example.

FIG. 27 is a schematic circuit diagram of a load driving device inaccordance with a twenty-third example.

FIG. 28 is a schematic circuit diagram of a load driving device inaccordance with a twenty-fourth example.

FIG. 29 is a schematic circuit diagram of a load driving device inaccordance with a twenty-fifth example.

FIG. 30 is a schematic circuit diagram of a load driving device inaccordance with a twenty-sixth example.

FIG. 31 is a schematic circuit diagram of a load driving device inaccordance with a twenty-seventh example.

FIG. 32 is a schematic circuit diagram of a load driving device inaccordance with a twenty-eighth example.

FIG. 33 is a schematic circuit diagram of a load driving device inaccordance with a twenty-ninth example.

FIG. 34 is a schematic circuit diagram of a load driving device inaccordance with a thirtieth example.

FIG. 35 is a schematic circuit diagram of a load driving device inaccordance with a thirty-first example.

FIG. 36 is a schematic circuit diagram of a load driving device inaccordance with a thirty-second example.

FIG. 37 is a schematic circuit diagram of a load driving device inaccordance with a thirty-third example.

FIG. 38 is a schematic circuit diagram of a load driving device inaccordance with a thirty-fourth example.

FIG. 39 is a schematic circuit diagram of a load driving device inaccordance with a thirty-fifth example.

FIG. 40 is a schematic circuit diagram of a load driving device inaccordance with a thirty-sixth example.

FIG. 41 is a schematic circuit diagram of a load driving device inaccordance with a thirty-seventh example.

FIG. 42 is a schematic circuit diagram of a load driving device inaccordance with a thirty-eighth example.

FIG. 43 is a schematic circuit diagram of a load driving device inaccordance with a thirty-ninth example.

FIG. 44 is a schematic circuit diagram of a load driving device inaccordance with a fortieth example.

FIG. 45 is a schematic circuit diagram of a load driving device inaccordance with a forty-first example.

FIG. 46 is a schematic circuit diagram of a load driving device inaccordance with a forty-second example.

FIG. 47 is a schematic circuit diagram of a load driving device inaccordance with a forty-third example.

FIG. 48 is a schematic circuit diagram of a load driving device inaccordance with a forty-fourth example.

FIG. 49 is a schematic circuit diagram of a load driving device inaccordance with a forty-fifth example.

FIG. 50 is a schematic circuit diagram of a load driving device inaccordance with a forty-sixth example.

FIG. 51 is a schematic circuit diagram of a load driving device inaccordance with a forty-seventh example.

FIG. 52 is a schematic circuit diagram of a load driving device inaccordance with a forty-eighth example.

FIG. 53 is a schematic circuit diagram of a load driving device inaccordance with a forty-ninth example.

FIG. 54 is a schematic circuit diagram of a load driving device inaccordance with a fiftieth example.

FIG. 55 is a schematic circuit diagram of a load driving device inaccordance with a fifty-first example.

FIG. 56 is a schematic circuit diagram of a load driving device inaccordance with a fifty-second example.

FIG. 57 is a schematic circuit diagram of a load driving device inaccordance with a fifty-third example.

FIG. 58 is a schematic circuit diagram of a load driving device inaccordance with a fifty-fourth example.

FIG. 59 is a schematic circuit diagram of a load driving device inaccordance with a fifty-fifth example.

FIG. 60 is a schematic circuit diagram of a load driving device inaccordance with a fifty-sixth example.

FIG. 61 is a schematic circuit diagram of a load driving device inaccordance with a fifty-seventh example.

FIG. 62 is a schematic circuit diagram of a load driving device inaccordance with a fifth-eighth example.

FIG. 63 is a schematic circuit diagram of a load driving device inaccordance with a fifty-ninth example.

FIG. 64 is a view showing an example of the application to a switchingpower supply circuit of step-down voltage type.

FIG. 65 is a view showing an example of the application to a switchingpower supply circuit of step-up voltage type.

FIG. 66 is a view showing an example of the application to a switchingpower supply circuit of inverting type.

FIG. 67 is a view showing an example of the application to a switchingpower supply circuit of step-up/down voltage type of REGSEPIC type.

FIG. 68 is a view showing an example of the application to a switchingpower supply circuit of step-up/down voltage type of SEPIC type.

FIG. 69 is a view showing an example of the application to a switchingpower supply circuit of transformer type (forward method).

FIG. 70 is a block diagram showing an electronic device comprising myload driving device.

FIG. 71 is a waveform chart showing one example of PWM control.

DETAILED DESCRIPTION First Example

My devices and apparatus will now be described in detail by way ofexample with reference to the accompanying drawings. FIG. 1 is aschematic circuit diagram of a load driving device in accordance with afirst example.

As shown in FIG. 1, a switching power supply circuit 100 is a voltagestep-up type switching power supply circuit for stepping up an input dcvoltage Vcc (referred to as input voltage) to provide a stepped up dcoutput voltage Vo1.

A coil L1 and a switch Q1 in the form of N-type MOS transistor areconnected in series between the power supply voltage Vcc and the ground.The voltage at node A of the coil L1 and switch Q1 is rectified by arectifying diode D1 and smoothed by a smoothing capacitor C1. Thesmoothed voltage is provided as the output voltage Vo1. In what followsvoltages represent potentials relative to the ground unless otherwisestated.

Connected in series between a terminal point P1 having the outputvoltage Vo1 and the ground is an external load 10 and a constant-currentsource I1. The operating point of the external load 10 depends on themagnitude of the current that flows through it. The external load 10 isprovided with a drive current Io having a predetermined magnitude set bythe constant-current source I1. The voltage generated at one terminal P2of the constant-current source I1 is taken as a detection voltage Vdet.

A control circuit Cont receives the detection voltage Vdet and areference voltage Vref from a reference voltage source B1, and generatesa switching signal for controlling the switching of the switch Q1 toequalize the detection voltage Vdet to the reference voltage Vref. Inthe example shown herein, the control circuit Cont includes an erroramplifier Eamp for amplifying the difference between the referencevoltage Vref and the detection voltage Vdet, and apulse-width-modulation (PWM) control circuit Pwm for generating a PWMsignal based on the output of the error amplifier Eamp. The PWM signalis provided as the switching signal.

The external load 10 is connected between the terminals P1 and P2. Thedevice may incorporate such external load in a portable electronicapparatus. In that example, the terminals P1 and P2 may be omitted.

An example of the external load 10 is a set of light emitting diodes(LEDs) LED1-LED3. In the example shown herein, the LEDs are white LEDs,which are used, for example, in a liquid crystal display (LCD) panel oras a backlight of a key. Although only three serial LEDs are shown inFIG. 1, the device may encompass more than three LEDs connected indifferent configurations (serial, parallel, or combination of serial andparallel connections) depending on the luminance required and the areato be illuminated.

The If-Vf characteristic of a white LED is shown in FIG. 2, where Ifstands for the current flowing through the LED and Vf for the voltageapplied to the LED. In FIG. 2, the characteristic curve is plotted on asemi-logarithmic scale with the abscissa representing current If inlogarithm and the coordinate representing voltage Vf. This LED emitslight when current If is in a broad range (e.g., from 1.5 mA (point B)to 20 mA (point A)). As current If is varied, the luminance of the LEDchanges, in accordance with the magnitude of current If.

When current If is 20 mA (point A), the LED is activated by voltage Vfof 3.4 V applied in forward direction. However, not all of the LEDsnecessarily have the same characteristic. For example, forwardactivation voltage Vf can differ from one LED to another in the rangefrom about 3.4 V to about 4.0 V when current If is 20 mA. As seen inthis example, white LEDs generally have higher forward activationvoltage Vf than LEDs of other colors. To activate three white LEDs inseries, the output voltage Vo1 must be at least 12.0 V or more.

FIG. 3 shows an exemplary circuit arrangement of a constant-currentsource I1. As shown in FIG. 3, a constant-current circuit I11 and anN-type MOS transistor (hereinafter referred to as N-type transistor) Q2are connected in series with each other between a power supply voltageVcc and the ground. The drain and the gate of this N-type transistor Q2directly connect together. In addition to the N-type transistor Q2, afurther N-type transistor Q3 having higher driving capability than theN-type transistor Q2 is provided to flow the drive current Io. The gateof the N-type transistor Q2 on the input-side is connected to the gateof the N-type transistor Q3 on output-side to form a current mirrorcircuit.

In FIG. 3, the magnitude of the drive current Io flowing through theN-type transistor Q3 may be arbitrarily set to a preferred value. Thiscan be done by adjusting the magnitude of the current flowing throughthe constant-current circuit I11.

Referring back to FIG. 1 again, the constant-current source I1 canperform constant-current operation if it is impressed with a voltagehigher than its saturation voltage of about 0.3 V, for example (which isthe saturation voltage of the N-type transistor Q3 of FIG. 3). Theportion of the voltage exceeding the saturation voltage (about 0.3 V),which is not necessary as the drive current, results in a power loss(being equal to voltage×current) inside the constant-current source I1.The output voltage Vo1 of the power supply circuit 100 is controlled toequalize the voltage drop Vdet across the constant-current source I1 tothe reference voltage Vref. Therefore, the reference voltage Vref is setto a level slightly higher than the saturation voltage (about 0.3 V) ofthe transistor used in the constant-current source I1.

Operation of the load driving device thus configured will now bedescribed with further reference to FIG. 4 showing the drivecurrent-output voltage characteristic of the driving device. First, themagnitude of the drive current Io to be passed through the LEDs of theload 10 is set for the constant-current circuit I11. Then on-offswitching operation of the switch Q1 is started in the switching powersupply circuit 100. This causes the output voltage Vo1 to risegradually.

As a consequence, the detection voltage Vdet will become equal to thereference voltage Vref, thereby causing the drive current Io to flowthrough the LEDs LED1-LED3 of the load 10. The LEDs will be activated toemit light at the predetermined luminance.

It should be appreciated that even if the forward voltage Vfcharacteristic varies from one LED to another for the LEDs LED1-LED3,only the output voltage Vo1 deviates from a predetermined value, withoutaffecting the luminance of the LEDs LED1-LED3. The detection voltageVdet, which represents the voltage drop across the constant-currentsource I1 is fixed. Hence, the output voltage Vo1 is equal to theconstant detection voltage Vdet plus the voltage drop Vled (=3×Vf)across the LEDs LED1-LED3 in accord with the drive current Io at thattime.

If the luminance of the LEDs LED1-LED3 needs to be changed, magnitude ofthe drive current Io may be changed. For example, if the drive currentIo is increased, the luminance of the LEDs LED1-LED3 will increaseaccordingly. With this increase in the drive current Io, the voltagedrop Vled across the LEDs LED1-LED3 becomes larger, in accordance withthe Io-Vo1 characteristic shown in FIG. 2. The slope of the Vo1-line ofFIG. 4 depends on the If-Vf characteristic shown in FIG. 2.

Since the voltage drop Vled across the LEDs LED1-LED3 increases inaccord with the increase in the drive current Io, the output voltage Vo1increases as shown by the characteristic curve of FIG. 4. However, sincethe detection voltage Vdet is fixed, the loss of power in theconstant-current source I1 does not increase any further even if thedrive current is increased to enhance the luminance. Thus, the loaddriving device maintains a high operating efficiency.

Second Example

FIG. 5 shows a circuit structure of a load driving device in accordancewith a second example. As shown in FIG. 5, the load driving device has afurther load 20 in addition to the forgoing load 10. Furthermore, aconstant-current source I20 is provided in association with the load 20.It should be understood that more than two loads can be added.

In the arrangement shown in FIG. 5, a constant-current source I10 isconnected in series with the load 10, through which flows a drivecurrent Io1. The voltage drop across the constant-current source I10 isutilized as a first detection voltage Vdet1. Similarly, aconstant-current source I20 is connected in series with the load 20,through which flows a drive current Io2. The voltage drop across theconstant-current source I20 is used as the second detection voltageVdet2. Symbols P11, P12, P21, and P22 indicate terminals for connectionwith the loads.

An error amplifier Eamp of the control circuit Cont has twonon-inverting input terminals (+) and one inverting input terminal (−).The two non-inverting input terminals (+) are fed with a first detectionvoltage Vdet1 and a second detection voltage Vdet2, one for eachterminal, while the inverting input terminal (−) is fed with thereference voltage Vref. In the error amplifier Eamp, the lower one ofthe first detection voltage Vdet1 and the second detection voltage Vdet2is compared with the reference voltage Vref. The rest of the circuitstructure is the same as that of the first example shown in FIG. 1.

The load driving device of FIG. 5 can adjust the individual drivecurrents Io1 and Io2 independently. The lower one of the voltage dropsVdet1 and Vdet2 of the constant-current sources I10 and I20,respectively, is automatically selected in the controlled switchingoperation performed by the power supply circuit 100, thereby securingoperations of the constant-current sources I10 and I20 providing theconstant drive current Io1 and Io2 to the multiple loads 10 and 20.

Thus, the second example provides the same merits as the first ifmultiple loads are involved.

Third Example

FIG. 6 shows a circuit structure of a load driving device in accordancewith a third example. As seen in FIG. 6, the switching power supplycircuit 100 has the same configuration as the one shown in FIG. 1.

In the third example, connected in series between a node providing anoutput voltage Vo and the ground are a first external load (referred toas first load) 10 driven by a predetermined constant current, avariable-resistance means in the form of N-type transistor Q2 havingvariable resistance in response to a control signal, and a resistor R1serving as a current detection means. The first load 10 is a load havingan operating point that depends on the magnitude of the current flowingthrough it. In this example, the load 10 is provided with the drivecurrent Io of a predetermined magnitude. The voltage drop across theresistor R1 is used as the first detection voltage Vdet1.

The control circuit Cont is fed with the first detection voltage Vdet1along with a first reference voltage Vref1 from a reference voltagesource B1.

The first load 10 is the same as the load 10 of FIG. 1. Connectedbetween a node having the output voltage Vo and the ground is a secondexternal load (referred to as second load) 20 driven by a voltage higherthan the predetermined voltage V1.

A voltage dividing circuit consisting of resistors R2 and R3 is providedto detect the output voltage Vo. One of the divided voltages serves asthe second detection voltage Vdet2. An error amplifier EA is provided atthe non-inverting input terminal (+) thereof with the second detectionvoltage Vdet2, and at the inverting input terminal (−) thereof with thesecond reference voltage Vref2 received from a reference voltage sourceB2. The second detection voltage Vdet2 and the second reference voltageVref2 are compared in the error amplifier EA to generate a controlsignal in accord with the difference between them. The control signal issupplied to the gate of the N-type transistor Q2.

To drive the second load 20 at the voltage higher than the predeterminedvoltage V1, the second reference voltage Vref2 is set to the voltageV1×R3/(R2+R3) obtained by dividing the predetermined voltage V1 by theresistors R2 and R3. As a result, when the output voltage Vo is higherthan the predetermined voltage V1, the N-type transistor Q2 is switchedon and has an extremely small resistance. That means that the transistorQ2 is virtually short-circuited (or in a low-resistance state). On theother hand, when the output voltage Vo becomes lower than thepredetermined voltage V1, the resistance of the N-type transistor Q2becomes high. Thus, the N-type transistor Q2 functions as avariable-resistance means, i.e., means for varying its resistance inresponse to a control signal.

Operation of the load driving device thus configured will now bedescribed with additional reference to FIG. 7 showing the Io-Vocharacteristic of the device, where Io and Vo stand for the drivecurrent and output voltage, respectively. In this load driving device,the output voltage Vo is maintained at the predetermined voltage V1 whenthe drive current Io is less than a predetermined magnitude Io1, asshown in FIG. 7. On the other hand, when the drive current Io exceedsthe predetermined magnitude Io1, the output voltage Vo increases inaccord with the increase in the drive current Io.

In operation, the first reference voltage Vref1 (=Io×R1) is set up inaccord with the magnitude of the drive current Io to be supplied to theLEDs of the first load 10. Suppose now that the drive current Io is setlarger than the predetermined magnitude Io1.

Then controlled on-off switching operation of the switch Q1 is startedin the switching power supply circuit 100 such that the first detectionvoltage Vdet1 becomes equal to the first reference voltage Vref1. Theswitching operation causes the output voltage Vo to rise gradually.

While the output voltage Vo is less than the predetermined voltage V1,the second detection voltage Vdet2 is smaller than the second referencevoltage Vref2. As a consequence, the N-type transistor Q2 will not beturned on, thereby sustaining a high resistance. Hence, the drivecurrent Io is insufficient to the load, and the first detection voltageVdet1 is lower than the first reference voltage Vref1, causing theoutput voltage Vo to rise gradually.

The rise of the output voltage Vo eventually equalizes the firstdetection voltage Vdet1 to the first reference voltage Vref1. Under thiscondition, the intended drive current Io flows through the LEDsLED1-LED3 of the first load 10, thereby activating the LEDs to emitlight with a predetermined luminance.

Under this condition, if the luminance characteristic of the LEDsLED1-LED3 fluctuates from one LED to another, the output voltage Vodeviates from a predetermined value due to the fluctuations, butluminance of the LEDs LED1-LED3 will be little affected. As aconsequence, the output voltage Vo becomes equal to the first detectionvoltage Vdet1 (which equals the first reference voltage Vref1) plus thevoltage drop Vled (=3×Vf) across the LEDs LED1-LED3 driven by the drivecurrent Io.

The output voltage Vo is then larger than the predetermined voltage V1.Hence, the second detection voltage Vdet2 derived from the outputvoltage Vo through voltage division is larger than the second referencevoltage Vref2. Under this condition, the N-type transistor Q2 is in ONstate under the control of the control signal received from the erroramplifier EA. The resistance of the N-type transistor Q2 under thiscondition is extremely small and it can be said that the transistor Q2is virtually short-circuited.

To make the luminance of the LEDs LED1-LED3 larger, the first referencevoltage Vref1 may be raised, which in turn increases the drive currentIo. With the drive current Io increased, luminance of the LEDs LED1-LED3will be enhanced more. The voltage drop Vled across the LEDs LED1-LED3also becomes larger, in accordance with the characteristic shown in FIG.2. The slope of the output voltage Vo shown in FIG. 7 is determined bythe If-Vf characteristic of FIG. 2.

Since the voltage drop Vled across the LEDs LED1-LED3 increases inaccord with the increase in the drive current Io, the output voltage Voincreases in accord with Io as shown in FIG. 7.

Conversely, to decrease the luminance of the LEDs LED1-LED3, the firstreference voltage Vref1 may be lowered to reduce the drive current Io.As the drive current Io is reduced, the luminance of the LEDs LED1-LED3decreases accordingly. Then the voltage drop Vled across the LEDsLED1-LED3 also decreases in accord with the If-Vf characteristic shownin FIG. 2.

If the drive current Io is set to a smaller magnitude than thepredetermined current magnitude Io1, the voltage drop Vled across theLEDs LED1-LED3 will be smaller accordingly. The output voltage Vo thentends to decrease below the predetermined voltage V1.

However, the second detection voltage Vdet2 then becomes equal to orlower than the second reference voltage Vref2. As a result, theresistance Rs of the N-type transistor Q2 increases in response to thecontrol signal received from the error amplifier EA.

With the increase in the resistance Rs of the N-type transistor Q2,drive current Io decreases and so does the first detection voltageVdet1. The power supply circuit 100 operates such that the firstdetection voltage Vdet1 becomes equal to the first reference voltageVref1. Then, the output voltage Vo rises by a magnitude equal to thevoltage drop Io×Rs across the N-type transistor Q2, which is the productof the drive current Io and the resistance Rs of the N-type transistorQ2.

As a result, when the drive current Io is set to a smaller magnitudethan predetermined magnitude Io1, the N-type transistor Q2 functions asa variable-resistance means of maintaining the output voltage Vo at thepredetermined voltage V1.

Although a voltage drop Io×Rs is induced by the N-type transistor Q2,the second load 20 is provided with the output voltage Vo larger thanthe predetermined voltage V1.

Fourth Example

FIG. 8 shows a circuit structure of a load driving device in accordancewith a fourth example. As seen in FIG. 8, this example lacks the N-typetransistor Q2 serving as a variable-resistance means, error amplifier EAto control the N-type transistor Q2, and reference voltage source B2 ofFIG. 6. However, the example has a three-input type error amplifier Eampsubstituting for the two-input type error amplifier of FIG. 6.

A first non-inverting input terminal (+) of the error amplifier Eamp isfed with the first detection voltage Vdet1, and a second non-invertinginput terminal (+) is fed with the second detection voltage Vdet2. Theinverting input terminal (−) of the error amplifier Eamp is fed with thefirst reference voltage Vref1.

This error amplifier Eamp automatically selects the lowest one of thefirst and second detection voltages Vdet1 and Vdet2, respectively,inputted to the first and second non-inverting input terminals (+),respectively, and compares the selected one with the first referencevoltage Vref1.

As shown in FIG. 8, the resistor R1 of FIG. 6 serving as a drive currentdetection means is replaced by an adjustable-current typeconstant-current circuit I1. This constant-current source I1 is the sameas one shown in FIG. 3. In this arrangement too, the output voltage Voof the power supply circuit is controlled to equalize the firstreference voltage Vdet1 (representing the voltage drop across theconstant-current source I1) to the first reference voltage Vref.Therefore, the reference voltage Vref1 is set to a level slightly largerthan the saturation voltage (about 0.3 V) of the transistor used in theconstant-current source I1.

On the other hand, the voltage division ratio of the resistors R2 and R3is set such that the second detection voltage Vdet2 balances the firstreference voltage Vref1 when the output voltage Vo has the predeterminedvoltage V1, i.e., V1×R3/(R2+R3)=Vref1.

The rest of the circuit structure of FIG. 8 is the same as that of thethird example shown in FIG. 6.

In the example shown in FIG. 8, the lower one of the first detectionvoltage Vdet1 representing the voltage drop across the constant-currentsource I1 and the second detection voltage Vdet2 obtained by voltagedivision of the output voltage Vo is automatically chosen in thecontrolled switching operation performed by the power supply circuit100.

The load driving device shown in FIG. 8 also provides the same outputcharacteristic as the third example shown in FIG. 6. This can be seen asfollows. Referring to FIG. 7, there is shown Io-Vo characteristic of theload driving device, in which the output voltage Vo is maintained at thepredetermined voltage V1 when the drive current Io is less than thepredetermined magnitude Io1, but the output voltage Vo increases withthe drive current Io if the drive current Io exceeds the predeterminedmagnitude Io1.

The resistor R1 serving as the current detection means in thearrangement of FIG. 6 may be replaced by the constant-current source I1of FIG. 5. In this example, the constant-current source I1 is adapted toadjust the magnitude of the constant-current. The first referencevoltage Vref1 may be fixed.

Fifth Example

FIG. 9 is a view showing a configuration of a load driving device inaccordance with a fifth example. Basically, the load driving device ofthe fifth example has a similar configuration to the load driving deviceof the first example (see FIG. 1), and is characterized in that aresistance R1 for current detection is provided at a position of aconstant-current source I1 and a detection voltage Vdet is derived fromone end of the resistance R1, that it is such configured to allow areference voltage Vref to be arbitrarily adjustable by a referencevoltage source B1, that input polarity of an error amplifier Eamp isinverted from input polarity of the error amplifier of the firstexample, and that a transistor Q2 for a shutdown in case of abnormalityis provided between an external load 10 and the resistance R1. However,the input polarity of the error amplifier Eamp may correspond with theinput polarity of the error amplifier Eamp of the first example.

The load driving device of the fifth example can achieve similar effectto the load driving device of the first example, without using theconstant-current source I1. In addition, since a current value of adrive current Io flowing through the resistance R1 is finally maintainedat a fixed value corresponding to the reference voltage Vref by outputfeedback control of a control circuit Cont, the resistance R1 can berecognized as the constant-current source I1 in that sense.

In addition, the load driving device of the fifth example can forciblyshut down operation of the load driving device by turning off thetransistor Q2 in response to an abnormality protection signal (not shownin FIG. 9) and blocking a current pathway to the external load 10.

In addition, in the load driving device in accordance with the fifthexample, the reference voltage Vref is inputted to a noninverting inputterminal (+) of the error amplifier Eamp, and the detection voltage Vdetis inputted to an inverting input terminal (−) of the error amplifierEamp. In this example, the lower the detection voltage Vdet becomescompared to the reference voltage Vref, the higher the output voltagelevel of the error amplifier Eamp becomes, and as the detection voltageVdet is closer to the reference voltage Vref, the output voltage levelof the error amplifier Eamp becomes lower.

Thus, a pulse-width modulation control circuit Pwm increases on duty ofa transistor Q1 when an output voltage of the error amplifier Eamp is ata higher level. On the other hand, the circuit may generate a gatesignal of the transistor Q1 to decrease on duty of the transistor Q1when the output voltage of the error amplifier Eamp is at a lower level.

Sixth Example

FIG. 10 is a view showing a configuration of a load driving device inaccordance with a sixth example. The load driving device of the sixthexample is characterized in that external loads 10 and 20 of multiplesystems are connected, that detection voltages Vdet1 and Vdet2 arederived from each one end of constant-current sources I10 and I20, thata selector SLT that selects any one of the detection voltages Vdet1 andVdet2 (one at a lower voltage level) and outputting it to an erroramplifier Eamp is provided, that the error amplifier Eamp has inputterminals of two systems (a noninverting input terminal (+) and aninverting input terminal (−)), and that output feedback circuits (anerror amplifier EA, a reference voltage source B2, and resistances R2and R3) that control continuity level of transistors Q21 and Q22 placedrespectively between the external loads 10 and 20 and theconstant-current sources I10 and I20, depending on an output voltage Vo,are provided.

In the example, a description of input polarity of the error amplifierEamp was given by taking an example of a configuration in which thereference voltage Vref1 is inputted into the noninverting terminal (+)and any one of the detection voltages Vdet1 and Vdet2 is inputted intothe inverting input terminal (−). However, a configuration is notlimited to this, and may be such that contrary to the above, a referencevoltage Vref1 is inputted to the inverting input terminal (−) and anyone of the detection voltages Vdet1 and Vdet2 is inputted into thenoninverting terminal (+). The input polarity of the error amplifierEamp can be arbitrarily selected in any example to be described in thefollowing, although this is not stated redundantly hereinafter. Hence,to clearly specify that such a modification can be made, as the inputpolarity of the error amplifier Eamp, first input polarity (refer to asign not parenthesized) and second input polarity (refer to aparenthesized sign) are included in FIGS. 10 to 63.

In addition, the abnormality protection transistor shown in FIG. 9 abovemay be inserted between the external loads 10 and constant-currentsource I10, and also inserted between the external loads 20 and theconstant-current source I20. The above abnormality protection transistormay also be inserted arbitrarily in any example to be described in thefollowing, although this is not stated redundantly hereinafter. Hence,to clearly specify that such a modification can be made, a position sdn(dash line circle) into which the abnormality protection transistor canbe inserted is indicated in FIGS. 10 to 63.

Seventh Example

FIG. 11 is a view showing a configuration of a load driving device inaccordance with a seventh example. The load driving device of theseventh example is characterized in that external loads 10 and 20 ofmultiple systems are connected, that detection voltages Vdet1 and Vdet2are derived from each one end of constant-current sources I10 and I20,that a selector SLT that selects any one of the detection voltages Vdet1and Vdet2 (one at a lower voltage level) and outputting it to an erroramplifier Eamp is provided, that the error amplifier Eamp has inputterminals of two systems (a noninverting input terminal (+) and aninverting input terminal (−)), and that output feedback circuits (erroramplifiers EA10 and EA20, and reference voltage sources B10 and B20)that control continuity level of transistors Q21 and Q22 placedrespectively between the external loads 10 and 20 and theconstant-current sources I10 and I20, depending on drive currents Io1and Io2 (more specifically, the detection voltages Vdet1 and Vdet2), areprovided.

Eighth Example

FIG. 12 is a view showing a configuration of a load driving device inaccordance with an eighth example. The load driving device of the eighthexample is characterized in that external loads 10 and 20 of multiplesystems are connected, that detection voltages Vdet1 and Vdet2 arederived from each one end of constant-current sources I10 and I20, thata selector SLT for selecting any one of the detection voltages Vdet1 andVdet2 (one at a lower voltage level) and outputting it to an erroramplifier Eamp is provided, and that the error amplifier Eamp has inputterminals of two systems (a noninverting input terminal (+) and aninverting input terminal (−)).

Ninth Example

FIG. 13 is a view showing a configuration of a load driving device inaccordance with a ninth example. The load driving device of the ninthexample is characterized in that external loads 10 and 20 of multiplesystems are connected, that as constant-current sources I10 and I20whose internal configuration different from FIG. 3 is adopted andpositions from which detection voltages Vdet1 and Vdet2 are derived havebeen changed, that a selector SLT for selecting any one of the detectionvoltages Vdet1 and Vdet2 (one at a lower voltage level) and outputtingit to an error amplifier Eamp is provided, that the error amplifier Eamphas input terminals of two systems (a noninverting input terminal (+)and an inverting input terminal (−)), and that output feedback circuits(an error amplifier EA, a reference voltage source B2, and resistancesR2 and R3) that control continuity level of transistors Q21 and Q22placed respectively between the external loads 10 and 20 and theconstant-current sources I10 and I20, depending on an output voltage Vo,are provided.

In the load driving device of the ninth example, the firstconstant-current source I10 has a pnp type bipolar transistor Qa,resistances Ra1 and Ra2, an operational amplifier Oa, and referencevoltage source Ba.

An emitter of the transistor QA connects to a terminal P12 (a cathode ofa light-emitting diode column, which forms an external load 10). Acollector of the transistor Qa connects to a ground end by way of aresistance Ra1. A base of the transistor Qa connects to a firstinverting input terminal (−) of the error amplifier Eamp. A resistanceRa2 is connected between the base and the emitter of the transistor Qa.

The inverting input terminal (−) of the operational amplifier Oaconnects to one end (derived end of a first sense voltage Vsa) of theresistance Ra1. The noninverting input terminal (+) of the operationalamplifier Oa connects to a positive electrode end (an end to which afirst reference voltage Vrefa is applied) of a first reference voltagesource Ba. A negative electrode end of the first reference voltagesource Ba connects to the ground. An output end of the operationalamplifier Oa connects to the base of the transistor Qa.

In addition, the second constant-current supply I20 has a pnp typebipolar transistor Qb, resistances Rb1 and Rb2, an operational amplifierOb, and a reference voltage source Bb.

An emitter of the transistor Qb is connected to a terminal P22 (cathodeof a light-emitting diode column, which forms an external load 20). Acollector of the transistor Qb is connected to a ground by way of theresistance Rb1. A base of the transistor Qb is connected to a secondinverting input terminal (−) of the error amplifier Eamp. The resistanceRb2 connects between the base and the emitter of the transistor Qb.

The inverting input terminal (−) of the operational amplifier Obconnects to one end of a resistance Rb1 (derived end of a second sensevoltage Vsb). The noninverting input terminal (+) of the operationalamplifier Ob connects to a positive electrode end (an end to which asecond reference voltage Vrefb is applied) of a second reference voltagesource Bb. A negative electrode end of the second reference voltagesource Bb connects to the ground. The output terminal of the operationalamplifier Ob is connected to the base of the transistor Qb.

In addition, the transistors Qa and Qb may be replaced with a P-channeltype MOS field-effect transistor, respectively. In that example,connections may be made such that the emitter is replaced with a source,the collector is replaced with a drain, and the base is replaced with agate.

In the first constant-current supply I10 of the above configuration, abase voltage of the transistor Qa, that is to say, the first detectionvoltage Vdet1 is controlled to generate a predetermined first drivecurrent Io1 so that a first sense voltage Vsa corresponds with a firstreference voltage Vrefa. In addition, if it is desired to adjust thefirst drive current Io1, the first reference voltage Vrefa may bevariably controlled arbitrarily.

Similarly, in the second constant-current supply I20 of the aboveconfiguration, a base voltage of the transistor Qb, that is to say, thesecond detection voltage Vdet2 is controlled to generate a predeterminedsecond drive current Io2 so that a second sense voltage Vsb correspondswith a second reference voltage Vrefb. In addition, if it is desired toadjust the second drive current Io2, the second reference voltage Vrefbmay be variably controlled arbitrarily.

If the above configuration is adopted, the first detection voltage Vdet1is not a voltage that appears at a connecting point (i.e., terminal P12)of the external load 10 and the first constant-current source I10 but avoltage from which a lowered voltage in the resistance Ra2 issubtracted. Similarly, the second detection voltage Vdet2 is not avoltage that appears at a connecting point (i.e., terminal P22) of theexternal load 20 and the second constant-current source I20, but avoltage from which lowered voltage in the resistance Rb2 is subtracted.

Thus, the load driving device of the ninth example can achieve effectsimilar to that described above, even in the configuration in which thepositions where the first detection voltage Vdet1 and the seconddetection voltage Vdet2 are derived are changed.

In other words, it is important that the load driving device has, as itscomponents, a power supply circuit that supplies an output voltage,which is converted from an input voltage, to load, a detection voltagegeneration circuit that generates a detection voltage which variesdepending on a magnitude of lowered voltage across the load, and acontrol circuit that controls the power supply circuit so that outputfeedback control of the output voltage is performed based on thedetection voltage, and various changes may be made to a method ofgenerating the detection voltage or a position where it is derived. Itcan be said that such a modification is included in the technologicalscope of any load driving devices disclosed herein.

Tenth Example

FIG. 14 is a view showing a configuration of a load driving device inaccordance with a tenth example. The load driving device of the tenthexample is characterized in that external loads 10 and 20 of multiplesystems are connected, that as constant-current sources I10 and I20, aninternal configuration different from FIG. 3 (a configuration similar tothe ninth example) is adopted and positions from which detectionvoltages Vdet1 and Vdet2 are derived have been changed, that a selectorSLT for selecting any one of the detection voltages Vdet1 and Vdet2 (oneat a lower voltage level) and outputting it to an error amplifier Eampis provided, that the error amplifier Eamp has input terminals of twosystems (a noninverting input terminal (+) and an inverting inputterminal (−)), and that output feedback circuits (error amplifiers EA10and EA20, and reference voltage sources B10 and B20) for controllingcontinuity level of transistors Q21 and Q22 placed respectively betweenthe external loads 10 and 20 and the constant-current sources I10 andI20, depending on drive currents Io1 and Io2 (more specifically, sensevoltages Vsa and Vsb), are provided.

Eleventh Example

FIG. 15 is a view showing a configuration of a load driving device inaccordance with an eleventh example. The load driving device of theeleventh example is characterized in that external loads 10 and 20 ofmultiple systems are connected, that as constant-current sources I10 andI20, an internal configuration different from FIG. 3 (a configurationsimilar to the ninth example) is adopted and positions from whichdetection voltages Vdet1 and Vdet2 are derived have been changed, that aselector SLT for selecting any one of the detection voltages Vdet1 andVdet2 (one at a lower voltage level) and outputting it to an erroramplifier Eamp is provided, and that the error amplifier Eamp has inputterminals of two systems (a noninverting input terminal (+) and aninverting input terminal (−)).

Twelfth Example

FIG. 16 is a view showing a configuration of a load driving device inaccordance with a twelfth example. The load driving device of thetwelfth example is characterized in that external loads 10 and 20 ofmultiple systems are connected, that resistances R10 and R20 for currentdetection are provided at positions of constant-current sources I10 andI20 and detection voltages Vdet1 and Vdet2 are derived from one end ofeach, that a selector SLT for selecting any one of the detectionvoltages Vdet1 and Vdet2 (one at a lower voltage level) and outputtingit to an error amplifier Eamp is provided, that the error amplifier Eamphas input terminals of two systems (a noninverting input terminal (+)and an inverting input terminal (−)), and that output feedback circuits(an error amplifier EA, a reference voltage source B2, and resistancesR2 and R3) for controlling continuity level of transistors Q21 and Q22placed respectively between the external loads 10 and 20 and theresistances R10 and R20, depending on an output voltage Vo, areprovided.

Thirteenth Example

FIG. 17 is a view showing a configuration of a load driving device inaccordance with a thirteenth example. The load driving device of thethirteenth example is characterized in that external loads 10 and 20 ofmultiple systems are connected, that resistances R10 and R20 for currentdetection are provided at positions of constant-current sources I10 andI20 and detection voltages Vdet1 and Vdet2 are derived from one end ofeach, that a selector SLT for selecting any one of the detectionvoltages Vdet1 and Vdet2 (one at a lower voltage level) and outputtingit to an error amplifier Eamp is provided, that the error amplifier Eamphas input terminals of two systems (a noninverting input terminal (+)and an inverting input terminal (−)), and that output feedback circuits(error amplifiers EA1 and EA20, and reference voltage sources B1 and B2)for controlling continuity level of transistors Q21 and Q22 placedrespectively between the external loads 10 and 20 and the resistancesR10 and R20, depending on drive currents Io1 and Io2 (more specifically,the detection voltages Vdet1 and Vdet2), are provided.

Fourteenth Example

FIG. 18 is a view showing a configuration of a load driving device inaccordance with a fourteenth example. The load driving device of thefourteenth example is characterized in that external loads 10 and 20 ofmultiple systems are connected, that resistances R10 and R20 for currentdetection are provided at positions of constant-current sources I10 andI20 and detection voltages Vdet1 and Vdet2 are derived from one end ofeach, that a selector SLT for selecting any one of the detectionvoltages Vdet1 and Vdet2 (one at a lower voltage level) and outputtingit to an error amplifier Eamp is provided, and that the error amplifierEamp has input terminals of two systems (a noninverting input terminal(+) and an inverting input terminal (−)).

Fifteenth Example

FIG. 19 is a view showing a configuration of a load driving device inaccordance with a fifteenth example. The load driving device of thefifteenth example is characterized in that an external load 10 of asingle system is connected, that a detection voltage Vdet1 is derivedfrom one end of a constant-current source I1, that an error amplifierEamp has input terminals of two systems (a noninverting input terminal(+) and an inverting input terminal (−)), and that output feedbackcircuits (an error amplifier EA, a reference voltage source B2, andresistances R2 and R3) for controlling continuity level of a transistorQ2 placed respectively between the external load 10 and theconstant-current source I1, depending on an output voltage Vo, areprovided.

Sixteenth Example

FIG. 20 is a view showing a configuration of a load driving device inaccordance with a sixteenth example. The load driving device of thesixteenth example is characterized in that an external load 10 of asingle system is connected, that a detection voltage Vdet is derivedfrom one end of a constant-current source I1, that an error amplifierEamp has input terminals of two systems (a noninverting input terminal(+) and an inverting input terminal (−)), and that output feedbackcircuits (an error amplifier EA, and a reference voltage source B2) forcontrolling continuity level of a transistor Q2 placed respectivelybetween the external load 10 and the constant-current source I1,depending on a drive current Io (more specifically, the detectionvoltage Vdet), are provided.

Seventeenth Example

FIG. 21 is a view showing a configuration of a load driving device inaccordance with a seventeenth example. The load driving device of theseventeenth example is characterized in that an external load 10 of asingle system is connected, that a detection voltage Vdet1 is derivedfrom one end of a constant-current source I1, and that an erroramplifier Eamp has input terminals of two systems (a noninverting inputterminal (+) and an inverting input terminal (−)).

Eighteenth Example

FIG. 22 is a view showing a configuration of a load driving device inaccordance with an eighteenth example. The load driving device of theeighteenth example is characterized in that an external load 10 of asingle system is connected, that as a constant-current source I1, aninternal configuration different from FIG. 3 (a configuration similar tothe first constant-current source I10 of the ninth example) is adoptedand a position from which a detection voltages Vdet1 is derived has beenchanged, that an error amplifier Eamp has input terminals of two systems(a noninverting input terminal (+) and an inverting input terminal (−)),and that output feedback circuits (an error amplifier EA, a referencevoltage source B2, and resistances R2 and R3) for controlling continuitylevel of a transistor Q2 placed respectively between the external load10 and the constant-current source I1, depending on an output voltageVo, are provided.

Nineteenth Example

FIG. 23 is a view showing a configuration of a load driving device inaccordance with a nineteenth example. The load driving device of thenineteenth example is characterized in that an external load 10 of asingle system is connected, that as a constant-current source I1, aninternal configuration different from FIG. 3 (a configuration similar tothe first constant-current source I10 of the ninth example) is adoptedand a position from which a detection voltages Vdet1 is derived has beenchanged, that an error amplifier Eamp has input terminals of two systems(a noninverting input terminal (+) and an inverting input terminal (−)),and that output feedback circuits (an error amplifier EA, and areference voltage source B2) for controlling continuity level of atransistors Q2 between the external load 10 and the constant-currentsource I1, depending on a drive current Io (more specifically, a sensevoltage Vsa), are provided.

Twentieth Example

FIG. 24 is a view showing a configuration of a load driving device inaccordance with a twentieth example. The load driving device of thetwentieth example is characterized in that an external load 10 of asingle system is connected, that as a constant-current source I1, aninternal configuration different from FIG. 3 (a configuration similar tothe first constant-current source I10 of the ninth example) is adoptedand a position from which a detection voltages Vdet is derived has beenchanged, that an error amplifier Eamp has input terminals of two systems(a noninverting input terminal (+) and an inverting input terminal (−)).

Twenty-First Example

FIG. 25 is a view showing a configuration of a load driving device inaccordance with a twenty-first example. The load driving device of thetwenty-first example is characterized in that an external load 10 of asingle system is connected, that a resistance R1 for current detectionis provided at a position of a constant-current source I1 and adetection voltage Vdet1 is derived from one end of the resistance R1,that an error amplifier Eamp has input terminals of two systems (anoninverting input terminal (+) and an inverting input terminal (−)),and that output feedback circuits (an error amplifier EA, a referencevoltage source B2, and resistances R2 and R3) for controlling continuitylevel of a transistor Q2 between the external load 10 and the resistanceR1, depending on an output voltage Vo, are provided.

Twenty-Second Example

FIG. 26 is a view showing a configuration of a load driving device inaccordance with a twenty-second example. The load driving device of thetwenty-second example is characterized in that an external load 10 of asingle system is connected, that a resistance R1 for current detectionis provided at a position of a constant-current source I1 and adetection voltage Vdet is derived from one end of the resistance R1,that an error amplifier Eamp has input terminals of two systems (anoninverting input terminal (+) and an inverting input terminal (−)),and that output feedback circuits (an error amplifier EA, and areference voltage source B2) for controlling continuity level of atransistor Q2 between the external load 10 and the resistance R1,depending on a drive current Io (more specifically, the detectionvoltage Vdet), are provided.

Twenty-Third Example

FIG. 27 is a view showing a configuration of a load driving device inaccordance with a twenty-third example. The load driving device of thetwenty-third example is characterized in that an external load 10 of asingle system is connected, that a resistance R1 for current detectionis provided at a position of a constant-current source I1 and adetection voltage Vdet is derived from one end of the resistance R1, andthat an error amplifier Eamp has input terminals of two systems (anoninverting input terminal (+) and an inverting input terminal (−)).

Twenty-Fourth Example

FIG. 28 is a view showing a configuration of a load driving device inaccordance with a twenty-fourth example. The load driving device of thetwenty-fourth example is characterized in that an external load 10 of asingle system is connected, that a detection voltage Vdet1 is derivedfrom one end of a constant-current source I1, that an error amplifierEamp has input terminals of three systems (a noninverting input terminal(+), a first inverting input terminal (−), and a second inverting inputterminal (−)), that in the error amplifier Eamp, an error voltagecorresponding to any one (one at a lower voltage level) of the detectionvoltage Vdet1 and a detection voltage Vdet2 (divided voltage of anoutput voltage Vo) is generated, and that output feedback circuits (anerror amplifier EA, a reference voltage source B2, and resistances R2and R3) for controlling continuity level of a transistor Q2 between theexternal load 10 and the constant-current source I1, depending on anoutput voltage Vo, are provided.

Twenty-Fifth Example

FIG. 29 is a view showing a configuration of a load driving device inaccordance with a twenty-fifth example. The load driving device of thetwenty-fifth example is characterized in that an external load 10 of asingle system is connected, that a detection voltage Vdet1 is derivedfrom one end of a constant-current source I1, that an error amplifierEamp has input terminals of three systems (a noninverting input terminal(+), a first inverting input terminal (−), and a second inverting inputterminal (−)), that in the error amplifier Eamp, an error voltagecorresponding to any one (one at a lower voltage level) of the detectionvoltage Vdet1 and a detection voltage Vdet2 (divided voltage of anoutput voltage Vo) is generated, and that output feedback circuits (anerror amplifier EA and a reference voltage source B2) for controllingcontinuity level of a transistor Q2 between the external load 10 and theconstant-current source I1, depending on a drive current Io (morespecifically the detection voltage Vdet1), are provided.

Twenty-Sixth Example

FIG. 30 is a view showing a configuration of a load driving device of atwenty-sixth example. The load driving device of the twenty-sixthexample is characterized in that an external load 10 of a single systemis connected, that a detection voltage Vdet1 is derived from one end ofa constant-current source I1, that an error amplifier Eamp has inputterminals of three systems (a noninverting input terminal (+), a firstinverting input terminal (−), and a second inverting input terminal(−)), and that in the error amplifier Eamp, an error voltagecorresponding to any one (one at a lower voltage level) of the detectionvoltage Vdet1 and a detection voltage Vdet2 (divided voltage of anoutput voltage Vo) is generated.

Twenty-Seventh Example

FIG. 31 is a view showing a configuration of a load driving device inaccordance with a twenty-seventh example. The load driving device of thetwenty-seventh example is characterized in that external loads 10 and 20of multiple systems are connected, that detection voltages Vdet1 andVdet2 are derived from each one end of constant-current sources I10 andI20, that an error amplifier Eamp has input terminals of three systems(a noninverting input terminal (+), a first inverting input terminal(−), and a second inverting input terminal (−)), that in the erroramplifier Eamp, an error voltage corresponding to any one (one at alower voltage level) of the detection voltages Vdet1 and Vdet2 isgenerated, and that output feedback circuits (an error amplifier EA, areference voltage source B2, and resistances R2 and R3) for controllingcontinuity level of transistors Q21 and Q22 placed respectively betweenthe external loads 10 and 20 and the constant-current sources I10 andI20, depending on an output voltage Vo, are provided.

Twenty-Eighth Example

FIG. 32 is a view showing a configuration of a load driving device inaccordance with a twenty-eighth example. The load driving device of thetwenty-eighth example is characterized in that external loads 10 and 20of multiple systems are connected, that detection voltages Vdet1 andVdet2 are derived from each one end of constant-current sources I10 andI20, that an error amplifier Eamp has input terminals of three systems(a noninverting input terminal (+), a first inverting input terminal(−), and a second inverting input terminal (−)), that in the erroramplifier Eamp, an error voltage corresponding to any one (one at alower voltage level) of the detection voltages Vdet1 and Vdet2 isgenerated, and that output feedback circuits (error amplifiers EA10 andEA20, and reference voltage sources B10 and B20) for controllingcontinuity level of transistors Q21 and Q22 placed respectively betweenthe external loads 10 and 20 and the constant-current sources I10 andI20, depending on drive currents Io1 and Io2 (more specifically, thedetection voltages Vdet1 and Vdet2), are provided.

Twenty-Ninth Example

FIG. 33 is a view showing a configuration of a load driving device inaccordance with a twenty-ninth example. The load driving device of thetwenty-ninth example is characterized in that external loads 10 and 20of multiple systems are connected, that detection voltages Vdet1 andVdet2 are derived from each one end of constant-current sources I10 andI20, that an error amplifier Eamp has input terminals of three systems(a noninverting input terminal (+), a first inverting input terminal(−), and a second inverting input terminal (−)), and that in the erroramplifier Eamp, an error voltage corresponding to any one (one at alower voltage level) of the detection voltages Vdet1 and Vdet2 isgenerated.

Thirtieth Example

FIG. 34 is a view showing a configuration of a load driving device inaccordance with a thirtieth example. The load driving device of thethirtieth example is characterized in that an external load 10 of asingle system is connected, that as a constant-current source I1, aninternal configuration different from FIG. 3 (a configuration similar tothe first constant-current source I10 of the ninth example) is adoptedand a position from which a detection voltages Vdet1 is derived has beenchanged, that an error amplifier Eamp has input terminals of threesystems (a noninverting input terminal (+), a first inverting inputterminal (−), and a second inverting input terminal (−)), that in theerror amplifier Eamp, an error voltage corresponding to any one (one ata lower voltage level) of detection voltages Vdet1 and Vdet2 (dividedvoltage of an output voltage Vo) is generated, and that output feedbackcircuits (an error amplifier EA, a reference voltage source B2, andresistances R2 and R3) for controlling continuity level of a transistorQ2 between the external load 10 and the constant-current source I1,depending on an output voltage Vo, are provided.

Thirty-First Example

FIG. 35 is a view showing a configuration of a load driving device inaccordance with a thirty-first example. The load driving device of thethirty-first example is characterized in that an external load 10 of asingle system is connected, that as a constant-current source I1, aninternal configuration different from FIG. 3 (a configuration similar tothe first constant-current source I10 of the ninth example) is adoptedand a position from which a detection voltages Vdet1 is derived has beenchanged, that an error amplifier Eamp has input terminals of threesystems (a noninverting input terminal (+), a first inverting inputterminal (−), and a second inverting input terminal (−)), that in theerror amplifier Eamp, an error voltage corresponding to any one (one ata lower voltage level) of detection voltages Vdet1 and Vdet2 (dividedvoltage of an output voltage Vo) is generated, and that output feedbackcircuits (an error amplifier EA and a reference voltage source B2) forcontrolling continuity level of a transistor Q2 between the externalload 10 and the constant-current source I1, depending on a drive currentIo (more specifically, a sense voltage Vsa), are provided.

Thirty-Second Example

FIG. 36 is a view showing a configuration of a load driving device inaccordance with thirty-second example. The load driving device of thethirty-second example is characterized in that an external load 10 of asingle system is connected, that as a constant-current source I1, aninternal configuration different from FIG. 3 (a configuration similar tothe first constant-current source I10 of the ninth example) is adoptedand a position from which a detection voltages Vdet1 is derived has beenchanged, that an error amplifier Eamp has input terminals of threesystems (a noninverting input terminal (+), a first inverting inputterminal (−), and a second inverting input terminal (−)), and that inthe error amplifier Eamp, an error voltage corresponding to any one (oneat a lower voltage level) of detection voltages Vdet1 and Vdet2 (dividedvoltage of an output voltage Vo) is generated.

Thirty-Third Example

FIG. 37 is a view showing a configuration of a load driving device inaccordance with a thirty-third example. The load driving device of thethirty-third example is characterized in that external loads 10 and 20of multiple systems are connected, that as constant-current sources I10and I20, an internal configuration different from FIG. 3 (aconfiguration similar to the ninth example) is adopted and positionsfrom which detection voltages Vdet1 and Vdet2 are derived have beenchanged, that an error amplifier Eamp has input terminals of threesystems (a noninverting input terminal (+), a first inverting inputterminal (−), and a second inverting input terminal (−)), that in theerror amplifier Eamp, an error voltage corresponding to any one (one ata lower voltage level) of the detection voltages Vdet1 and Vdet2 isgenerated, and that output feedback circuits (an error amplifier EA, areference voltage source B20, and resistances R2 and R3) for controllingcontinuity level of transistors Q21 and Q22 placed respectively betweenthe external loads 10 and 20 and the constant-current sources I10 andI20, depending on an output voltage Vo, are provided.

Thirty-Fourth Example

FIG. 38 is a view showing a configuration of a load driving device inaccordance with a thirty-fourth example. The load driving device of thethirty-fourth example is characterized in that external loads 10 and 20of multiple systems are connected, that as constant-current sources I10and I20, an internal configuration different from FIG. 3 (aconfiguration similar to the ninth example) is adopted and positionsfrom which detection voltages Vdet1 and Vdet2 are derived have beenchanged, that an error amplifier Eamp has input terminals of threesystems (a noninverting input terminal (+), a first inverting inputterminal (−), and a second inverting input terminal (−)), that in theerror amplifier Eamp, an error voltage corresponding to any one (one ata lower voltage level) of the detection voltages Vdet1 and Vdet2 isgenerated, and that output feedback circuits (error amplifiers EA10 andEA20 and reference voltage sources B10 and B20) for controllingcontinuity level of transistors Q21 and Q22 placed respectively betweenthe external loads 10 and 20 and the constant-current sources I10 andI20, depending on drive currents Io1 and Io2 (more specifically, sensevoltages Vsa and Vsb), are provided.

Thirty-Fifth Example

FIG. 39 is a view showing a configuration of a load driving device inaccordance with a thirty-fifth example. The load driving device of thethirty-fifth example is characterized in that external loads 10 and 20of multiple systems are connected, that as constant-current sources I10and I20, an internal configuration different from FIG. 3 (aconfiguration similar to the ninth example) is adopted and positionsfrom which detection voltages Vdet1 and Vdet2 are derived have beenchanged, that an error amplifier Eamp has input terminals of threesystems (a noninverting input terminal (+), a first inverting inputterminal (−), and a second inverting input terminal (−)), and that inthe error amplifier Eamp, an error voltage corresponding to any one (oneat a lower voltage level) of the detection voltages Vdet1 and Vdet2 isgenerated.

Thirty-Sixth Example

FIG. 40 is a view showing a configuration of a load driving device of athirty-sixth example. The load driving device of the thirty-sixthexample is characterized in that an external load 10 of a single systemis connected, that a resistance R1 for current detection is provided ata position of a constant-current source I1 and a detection voltage Vdet1is derived from one end of the resistance R1, that an error amplifierEamp has input terminals of three systems (a noninverting input terminal(+), a first inverting input terminal (−), and a second inverting inputterminal (−)), that in the error amplifier Eamp, an error voltagecorresponding to any one (one at a lower voltage level) of the detectionvoltage Vdet1 and a detection voltage Vdet2 (divided voltage of anoutput voltage Vo) is generated, and that output feedback circuits (anerror amplifier EA, a reference voltage source B2, and resistances R2and R3) for controlling continuity level of a transistor Q2 between theexternal load 10 and the resistance R1, depending on an output voltageVo, are provided.

Thirty-Seventh Example

FIG. 41 is a view showing a configuration of a load driving device inaccordance with a thirty-seventh example. The load driving device of thethirty-seventh example is characterized in that an external load 10 of asingle system is connected, that a resistance R1 for current detectionis provided at a position of a constant-current source I1 and adetection voltage Vdet1 is derived from one end of the resistance R1,that an error amplifier Eamp has input terminals of three systems (anoninverting input terminal (+), a first inverting input terminal (−),and a second inverting input terminal (−)), that in the error amplifierEamp, an error voltage corresponding to any one (one at a lower voltagelevel) of the detection voltage Vdet1 and a detection voltage Vdet2(divided voltage of an output voltage Vo) is generated, and that outputfeedback circuits (an error amplifier EA, and a reference voltage sourceB20) for controlling continuity level of a transistor Q2 between theexternal load 10 and the resistance R1, depending on a drive current Io(more specifically, the detection voltage Vdet1), are provided.

Thirty-Eighth Example

FIG. 42 is a view showing a configuration of a load driving device inaccordance with a thirty-eighth example. The load driving device of thethirty-eighth example is characterized in that an external load 10 of asingle system is connected, that a resistance R1 for current detectionis provided at a position of a constant-current source I1 and adetection voltage Vdet1 is derived from one end of the resistance R1,that an error amplifier Eamp has input terminals of three systems (anoninverting input terminal (+), a first inverting input terminal (−),and a second inverting input terminal (−)), and that in the erroramplifier Eamp, an error voltage corresponding to any one (one at alower voltage level) of the detection voltage Vdet1 and a detectionvoltage Vdet2 (divided voltage of an output voltage Vo) is generated.

Thirty-Ninth Example

FIG. 43 is a view showing a configuration of a load driving device inaccordance with a thirty-ninth example. The load driving device of thethirty-ninth example is characterized in that external loads 10 and 20of multiple systems are connected, that resistances R10 and R20 forcurrent detection are provided at positions of constant-current sourcesI10 and I20 and detection voltages Vdet1 and Vdet2 are derived from oneend of each, that an error amplifier Eamp has input terminals of threesystems (a noninverting input terminal (+), a first inverting inputterminal (−), and a second inverting input terminal (−)), that in theerror amplifier Eamp, an error voltage corresponding to any one (one ata lower voltage level) of the detection voltages Vdet1 and Vdet2 isgenerated, and that output feedback circuits (an error amplifier EA10, areference voltage sources B2, and resistances R2 and R3) for controllingcontinuity level of transistors Q21 and Q22 placed respectively betweenthe external loads 10 and 20 and the resistances R10 and R20, dependingon an output voltage Vo, are provided.

Fortieth Example

FIG. 44 is a view showing a configuration of a load driving device inaccordance with a fortieth example. The load driving device of thefortieth example is characterized in that external loads 10 and 20 ofmultiple systems are connected, that resistances R10 and R20 for currentdetection are provided at positions of constant-current sources I10 andI20 and detection voltages Vdet1 and Vdet2 are derived from one end ofeach, that an error amplifier Eamp has input terminals of three systems(a noninverting input terminal (+), a first inverting input terminal(−), and a second inverting input terminal (−)), that in the erroramplifier Eamp, an error voltage corresponding to any one (one at alower voltage level) of the detection voltages Vdet1 and Vdet2 isgenerated, and that output feedback circuits (error amplifiers EA10 andEA20 and reference voltage sources B10 and B20) for controllingcontinuity level of transistors Q21 and Q22 placed respectively betweenthe external loads 10 and 20 and the resistances R10 and R20, dependingon drive currents Io1 and Io2 (more specifically, the detection voltagesVdet1 and Vdet2), are provided.

Forty-First Example

FIG. 45 is a view showing a configuration of a load driving device of aforty-first example. The load driving device of the forty-first exampleis characterized in that external loads 10 and 20 of multiple systemsare connected, that resistances R10 and R20 for current detection areprovided at positions of constant-current sources I10 and I20 anddetection voltages Vdet1 and Vdet2 are derived from one end of each,that an error amplifier Eamp has input terminals of three systems (anoninverting input terminal (+), a first inverting input terminal (−),and a second inverting input terminal (−)), and that in the erroramplifier Eamp, an error voltage corresponding to any one (one at alower voltage level) of the detection voltages Vdet1 and Vdet2 isgenerated.

Forty-Second Example

FIG. 46 is a view showing a configuration of a load driving device inaccordance with a forty-second example. The load driving device of theforty-second example is characterized in that an external load 10 of asingle system is connected, that a detection voltage Vdet1 is derivedfrom one end of a constant-current source I1, that the detection voltageVdet1 and a detection voltage Vdet2 (divided voltage of an outputvoltage Vo) are respectively inputted into separate error amplifiersEamp1 and Eamp2, that in pulse-width modulation control circuit Pwm, Onduty of a transistor Q1 is determined depending on any one output (oneat a higher output voltage level) of the error amplifiers Eamp1 andEamp2, and that output feedback circuits (an error amplifier EA, areference voltage source B3, and resistances R2 and R3) for controllingcontinuity level of a transistor Q2 between the external load 10 and theconstant-current source I1, depending on an output voltage Vo, areprovided.

The load driving device of the forty-second example can individually seta reference voltage Vref1 to be inputted into the error amplifier Eamp1and a reference voltage Vref2 to be inputted into the error amplifierEamp2. Thus, there is flexibility when characteristics of the multipleexternal loads 10, 20 differ.

Forty-Third Example

FIG. 47 is a view showing a configuration of a load driving device inaccordance with a forty-third example. The load driving device of theforty-third example is characterized in that an external load 10 of asingle system is connected, that a detection voltage Vdet1 is derivedfrom one end of a constant-current source I1, that the detection voltageVdet1 and a detection voltage Vdet2 (divided voltage of an outputvoltage Vo) are respectively inputted into separate error amplifiersEamp1 and Eamp2, that in pulse-width modulation control circuit Pwm, Onduty of a transistor Q1 is determined depending on any one output (oneat a higher output voltage level) of the error amplifiers Eamp1 andEamp2, and that output feedback circuits (an error amplifier EA and areference voltage source B3) for controlling continuity level of atransistor Q2 between the external load 10 and the constant-currentsource I1, depending on a drive current Io (more specifically, thedetection voltage Vdet1), are provided.

Forty-Fourth Example

FIG. 48 is a view showing a configuration of a load driving device inaccordance with a forty-fourth example. The load driving device of theforty-fourth example is characterized in that an external load 10 of asingle system is connected, that a detection voltage Vdet1 is derivedfrom one end of a constant-current source I1, that the detection voltageVdet1 and a detection voltage Vdet2 (divided voltage of an outputvoltage Vo) are respectively inputted into separate error amplifiersEamp1 and Eamp2, that in pulse-width modulation control circuit Pwm, Onduty of a transistor Q1 is determined depending on any one output (oneat a higher output voltage level) of the error amplifiers Eamp1 andEamp2.

Forty-Fifth Example

FIG. 49 is a view showing a configuration of a load driving device inaccordance with a forty-fifth example. The load driving device of theforty-fifth example is characterized in that external loads 10 and 20 ofmultiple systems are connected, that detection voltages Vdet1 and Vdet2are derived from each one end of constant-current sources I10 and I20,that the detection voltages Vdet1 and Vdet2 are respectively inputtedinto separate error amplifiers Eamp1 and Eamp2, that in pulse-widthmodulation control circuit Pwm, On duty of a transistor Q1 is determineddepending on any one output (one at a higher output voltage level) ofthe error amplifiers Eamp1 and Eamp2, and that output feedback circuits(an error amplifier EA, a reference voltage sources B3, and resistancesR2 and R3) for controlling continuity level of transistors Q21 and Q22placed respectively between the external loads 10 and 20 and theconstant-current sources I10 and I20, depending on an output voltage Vo,are provided.

Forty-Sixth Example

FIG. 50 is a view showing a configuration of a load driving device inaccordance with a forty-sixth example. The load driving device of theforty-sixth example is characterized in that external loads 10 and 20 ofmultiple systems are connected, that detection voltages Vdet1 and Vdet2are derived from each one end of constant-current sources I10 and I20,that the detection voltages Vdet1 and Vdet2 are respectively inputtedinto separate error amplifiers Eamp1 and Eamp2, that in pulse-widthmodulation control circuit Pwm, On duty of a transistor Q1 is determineddepending on any one output (one at a higher output voltage level) ofthe error amplifiers Eamp1 and Eamp2, and that output feedback circuits(error amplifiers EA10 and EA20 and reference voltage sources B10 andB20) for controlling continuity level of transistors Q21 and Q22 placedrespectively between the external loads 10 and 20 and theconstant-current sources I10 and I20, depending on drive currents Io1and Io2 (more specifically, the detection voltages Vdet1 and Vdet2), areprovided.

Forty-Seventh Example

FIG. 51 is a view showing a configuration of a load driving device inaccordance with a forty-seventh example. The load driving device of theforty-seventh example is characterized in that external loads 10 and 20of multiple systems are connected, that detection voltages Vdet1 andVdet2 are derived from each one end of constant-current sources I10 andI20, that the detection voltages Vdet1 and Vdet2 are respectivelyinputted into separate error amplifiers Eamp1 and Eamp2, that inpulse-width modulation control circuit Pwm, On duty of a transistor Q1is determined depending on any one output (one at a higher outputvoltage level) of the error amplifiers Eamp1 and Eamp2.

Forty-Eighth Example

FIG. 52 is a view showing a configuration of a load driving device inaccordance with a forty-eighth example. The load driving device of theforty-eighth example is characterized in that an external load 10 of asingle system is connected, that as a constant-current source I1, aninternal configuration different from FIG. 3 (a configuration similar tothe first constant-current source I10 of the ninth example) is adoptedand a position from which a detection voltages Vdet1 is derived has beenchanged, that detection voltages Vdet1 and Vdet2 (divided voltage of anoutput voltage Vo) are respectively inputted into separate erroramplifiers Eamp1 and Eamp2, that in pulse-width modulation controlcircuit Pwm, On duty of a transistor Q1 is determined depending on anyone output (one at a higher output voltage level) of the erroramplifiers Eamp1 and Eamp2, and that output feedback circuits (an erroramplifier EA, a reference voltage source B3, and resistances R2 and R3)for controlling continuity level of a transistor Q2 between the externalload 10 and the constant-current source I1, depending on an outputvoltage Vo, are provided.

Forty-Ninth Example

FIG. 53 is a view showing a configuration of a load driving device inaccordance with a forty-ninth example. The load driving device of theforty-ninth example is characterized in that an external load 10 of asingle system is connected, that as a constant-current source I1, aninternal configuration different from FIG. 3 (a configuration similar tothe first constant-current source I10 of the ninth example) is adoptedand a position from which a detection voltages Vdet1 is derived has beenchanged, that detection voltages Vdet1 and Vdet2 (divided voltage of anoutput voltage Vo) are respectively inputted into separate erroramplifiers Eamp1 and Eamp2, that in pulse-width modulation controlcircuit Pwm, On duty of a transistor Q1 is determined depending on anyone output (one at a higher output voltage level) of the erroramplifiers Eamp1 and Eamp2, and that output feedback circuits (an erroramplifier EA and a reference voltage source B3) for controllingcontinuity level of a transistor Q2 between the external load 10 and theconstant-current source I1, depending on a drive current Io (morespecifically, a sense voltage Vsa), are provided.

Fiftieth Example

FIG. 54 is a view showing a configuration of a load driving device inaccordance with a fiftieth example. The load driving device of thefiftieth example is characterized in that an external load 10 of asingle system is connected, that as a constant-current source I1, aninternal configuration different from FIG. 3 (a configuration similar tothe first constant-current source I10 of the ninth example) is adoptedand a position from which a detection voltages Vdet1 is derived has beenchanged, that detection voltages Vdet1 and Vdet2 (divided voltage of anoutput voltage Vo) are respectively inputted into separate erroramplifiers Eamp1 and Eamp2, and that in pulse-width modulation controlcircuit Pwm, On duty of a transistor Q1 is determined depending on anyone output (one at a higher output voltage level) of the erroramplifiers Eamp1 and Eamp2.

Fifty-First Example

FIG. 55 is a view showing a configuration of a load driving device inaccordance with a fifty-first example. The load driving device of thefifty-first example is characterized in that external loads 10 and 20 ofmultiple systems are connected, that as constant-current sources I10 andI20, an internal configuration different from FIG. 3 (a configurationsimilar to the ninth example) is adopted and positions from whichdetection voltages Vdet1 and Vdet2 are derived have been changed, thatthe detection voltages Vdet1 and Vdet2 are respectively inputted intoseparate error amplifiers Eamp1 and Eamp2, that in pulse-widthmodulation control circuit Pwm, On duty of a transistor Q1 is determineddepending on any one output (one at a higher output voltage level) ofthe error amplifiers Eamp1 and Eamp2, and that output feedback circuits(an error amplifier EA, a reference voltage source B3 and resistances R2and R3) for controlling continuity level of transistors Q21 and Q22placed respectively between the external loads 10 and 20 and theconstant-current sources I10 and I20, depending on an output voltage Vo,are provided.

Fifty-Second Example

FIG. 56 is a view showing a configuration of a load driving device inaccordance with a fifty-second example. The load driving device of thefifty-second example is characterized in that external loads 10 and 20of multiple systems are connected, that as constant-current sources I10and I20, an internal configuration different from FIG. 3 (aconfiguration similar to the ninth example) is adopted and positionsfrom which detection voltages Vdet1 and Vdet2 are derived have beenchanged, that the detection voltages Vdet1 and Vdet2 are respectivelyinputted into separate error amplifiers Eamp1 and Eamp2, that inpulse-width modulation control circuit Pwm, On duty of a transistor Q1is determined depending on any one output (one at a higher outputvoltage level) of the error amplifiers Eamp1 and Eamp2, and that outputfeedback circuits (error amplifiers EA10 and EA20 and reference voltagesources B10 and B20) for controlling continuity level of transistors Q21and Q22 placed respectively between the external loads 10 and 20 and theconstant-current sources I10 and I20, depending on drive currents Io1and Io2 (more specifically, sense voltages Vsa and Vsb), are provided.

Fifty-Third Example

FIG. 57 is a view showing a configuration of a load driving device inaccordance with a fifty-third example. The load driving device of thefifty-third example is characterized in that external loads 10 and 20 ofmultiple systems are connected, that as constant-current sources I10 andI20, an internal configuration different from FIG. 3 (a configurationsimilar to the ninth example) is adopted and positions from whichdetection voltages Vdet1 and Vdet2 are derived have been changed, thatthe detection voltages Vdet1 and Vdet2 are respectively inputted intoseparate error amplifiers Eamp1 and Eamp2, that in pulse-widthmodulation control circuit Pwm, On duty of a transistor Q1 is determineddepending on any one output (one at a higher output voltage level) ofthe error amplifiers Eamp1 and Eamp2.

Fifty-Fourth Example

FIG. 58 is a view showing a configuration of a load driving device inaccordance with a fifty-fourth example. The load driving device of thefifty-fourth example is characterized in that an external load 10 of asingle system is connected, that a resistance R1 for current detectionis provided at a position of a constant-current source I1 and adetection voltage Vdet1 is derived from one end of the resistance R1,that detection voltages Vdet1 and Vdet2 (divided voltage of an outputvoltage Vo) are respectively inputted into separate error amplifiersEamp1 and Eamp2, that in pulse-width modulation control circuit Pwm, Onduty of a transistor Q1 is determined depending on any one output (oneat a higher output voltage level) of the error amplifiers Eamp1 andEamp2, and that output feedback circuits (an error amplifier EA, areference voltage source B3, and resistances R2 and R3) for controllingcontinuity level of a transistor Q2 between the external load 10 and theresistance R1, depending on an output voltage Vo, are provided.

Fifty-Fifth Example

FIG. 59 is a view showing a configuration of a load driving device inaccordance with a fifty-fifth example. The load driving device of thefifty-fifth example is characterized in that an external load 10 of asingle system is connected, that a resistance R1 for current detectionis provided at a position of a constant-current source I1 and adetection voltage Vdet1 is derived from one end of the resistance R1,that the detection voltage Vdet1 and a detection voltage Vdet2 (dividedvoltage of an output voltage Vo) are respectively inputted into separateerror amplifiers Eamp1 and Eamp2, that in pulse-width modulation controlcircuit Pwm, On duty of a transistor Q1 is determined depending on anyone output (one at a higher output voltage level) of the erroramplifiers Eamp1 and Eamp2, and that output feedback circuits (an erroramplifier EA and a reference voltage source B3) for controllingcontinuity level of a transistor Q2 between the external load 10 and theresistance R1, depending on a drive current Io (more specifically, thedetection voltage Vdet1), are provided.

Fifty-Sixth Example

FIG. 60 is a view showing a configuration of a load driving device inaccordance with a fifty-sixth example. The load driving device of thefifty-sixth example is characterized in that an external load 10 of asingle system is connected, that a resistance R1 for current detectionis provided at a position of a constant-current source I1 and adetection voltage Vdet1 is derived from one end of the resistance R1,that the detection voltage Vdet1 and a detection voltage Vdet2 (dividedvoltage of an output voltage Vo) are respectively inputted into separateerror amplifiers Eamp1 and Eamp2, and that in pulse-width modulationcontrol circuit Pwm, On duty of a transistor Q1 is determined dependingon any one output (one at a higher output voltage level) of the erroramplifiers Eamp1 and Eamp2.

Fifty-Seventh Example

FIG. 61 is a view showing a configuration of a load driving device inaccordance with a fifty-seventh example. The load driving device of thefifty-seventh example is characterized in that external loads 10 and 20of multiple systems are connected, that resistances R10 and R20 forcurrent detection are provided at positions of constant-current sourcesI10 and I20 and detection voltages Vdet1 and Vdet2 are derived from oneend of each, that the detection voltages Vdet1 and Vdet2 arerespectively inputted into separate error amplifiers Eamp1 and Eamp2,that in pulse-width modulation control circuit Pwm, On duty of atransistor Q1 is determined depending on any one output (one at a higheroutput voltage level) of the error amplifiers Eamp1 and Eamp2, and thatoutput feedback circuits (an error amplifier EA, a reference voltagesource B3, and resistances R2 and R3) for controlling continuity levelof transistors Q21 and Q22 placed respectively between the externalloads 10 and 20 and the resistances R10 and R20, depending on an outputvoltage Vo, are provided.

Fifty-Eighth Example

FIG. 62 is a view showing a configuration of a load driving device inaccordance with a fifty-eighth example. The load driving device of thefifty-eighth example is characterized in that external loads 10 and 20of multiple systems are connected, that resistances R10 and R20 forcurrent detection are provided at positions of constant-current sourcesI10 and I20 and detection voltages Vdet1 and Vdet2 are derived from oneend of each, that the detection voltages Vdet1 and Vdet2 arerespectively inputted into separate error amplifiers Eamp1 and Eamp2,that in pulse-width modulation control circuit Pwm, On duty of atransistor Q1 is determined depending on any one output (one at a higheroutput voltage level) of the error amplifiers Eamp1 and Eamp2, and thatoutput feedback circuits (error amplifiers EA10 and EA20 and referencevoltage sources B10 and B20) for controlling continuity level oftransistors Q21 and Q22 placed respectively between the external loads10 and 20 and the resistances R10 and R20, depending on drive currentsIo1 and Io2 (more specifically, the detection voltages Vdet1 and Vdet2),are provided.

Fifth-Ninth Example

FIG. 63 is a view showing a configuration of a load driving device inaccordance with a fifty-ninth example. The load driving device of thefifty-ninth example is characterized in that external loads 10 and 20 ofmultiple systems are connected, that resistances R10 and R20 for currentdetection are provided at positions of constant-current sources I10 andI20 and detection voltages Vdet1 and Vdet2 are derived from one end ofeach, that the detection voltages Vdet1 and Vdet2 are respectivelyinputted into separate error amplifiers Eamp1 and Eamp2, and that inpulse-width modulation control circuit Pwm, On duty of a transistor Q1is determined depending on any one output (one at a higher outputvoltage level) of the error amplifiers Eamp1 and Eamp2.

Variations of Power Supply Circuits

In the first to fifty-ninth examples described above, although adescription was given by taking as an example a switching power supplycircuit 100 of boost type which raises an input voltage Vcc to generatean output voltage Vo1 or Vo, my principles can be applied to powersupply circuits in general which generate a detection voltage whichvaries depending on a magnitude of a forward dropping voltage of anexternal load and perform output feedback control of the output voltageon the basis of the detection voltage. That is to say, various changescan be made to an output format of the power supply circuit, and it canbe said that such modifications are included in a technical scope of theload driving devices disclosed herein. In the following, one example ofthe variations of the power supply circuits will be described brieflywith reference to the drawings.

FIG. 64 is a view showing an example of the application of a switchingpower supply circuit of step-down voltage type. The switching powersupply circuit of step-down voltage type shown in FIG. 64 has aP-channel type MOS field-effect transistor Qa, a coil La, a diode Da, acapacitor Ca, a constant-current source Ia, and a control circuit Cont,lowers an input voltage Vi to generate a desired output voltage Vo, andsupplies the output voltage Vo to an LED (external load).

FIG. 65 is a view showing an example of the application of a switchingpower supply circuit of step-up voltage type. The switching power supplycircuit of step-up voltage type shown in FIG. 65 has an N-channel MOSfield-effect transistor Qb, a coil Lb, a diode Db, a capacitor Cb, aconstant-current source Ib, and a control circuit Cont, boosts an inputvoltage Vi to generate a desired output voltage Vo, and supplies theoutput voltage Vo to an LED (external load).

FIG. 66 is a view showing an example of the application to a switchingpower supply circuit of inverting type. The switching power supplycircuit of inverting type shown in FIG. 66 has a P-channel type MOSfield-effect transistor Qc, a coil Lc, a diode Dc, a capacitor Cc, aconstant-current source Ic, and a control circuit Cont, inverts plus andminus of an input voltage Vi to generate a desired output voltage Vo,and supplies the output voltage Vo to an LED (external load).

FIG. 67 is a view showing an example of the application to a switchingpower supply voltage of step-up/down voltage type of REGSEPIC type. Theswitching power supply circuit of step-up/down voltage type shown inFIG. 67 has a P-channel type field-effect transistor Qd1, an N-channeltype MOS field-effect transistor Qd2, a coil Ld, a diode Dd, a capacitorCd, a constant-current source Id, and a control circuit Cont, boosts orlowers an input voltage Vi to generate a desired output voltage Vo, andsupplies the output voltage Vo to an LED (external load).

FIG. 68 is a view showing an example of the application to a switchingpower supply circuit of step-up/down voltage type of SEPIC (single endedprimary inductance converter) type. The switching power supply circuitof step-up/down voltage type shown in FIG. 68 has an N-channel type MOSfield-effect transistor Qe, a coil Le1 and a coil Le2, a diode De, acapacitor Ce1 and a capacitor Ce2, a constant-current source Ie, and acontrol circuit Cont, boosts or lowers an input voltage Vi to generate adesired output voltage Vo, and supplies the output voltage Vo to an LED(external load).

FIG. 69 is a view showing an example of the application to a switchingpower supply circuit of transformer type (forward method). The switchingpower supply circuit of transformer type (forward method) shown in FIG.69 has an N-channel type MOS field-effect transistor Qf, a transformerTf, a coil Lf, a diode Df1 and a diode Df2, a capacitor Cf, aconstant-current source If, a control circuit Cont, and a photocouplerPc, generates from an input voltage Vi a desired output voltage Vocorresponding to a winding ratio of the transformer Tf, and supplies theoutput voltage Vo to an LED (external load).

Any of the power supply device of various types shown in FIGS. 64 to 69generates a detection voltage Vdet which varies depending on a magnitudeof a forward dropping voltage of an LED (external load), and performsoutput feedback control of an output voltage Vo on the basis of thedetection voltage Vdet.

In addition, as a circuit block X (part surrounded by the chaindouble-dashed line) shown in FIGS. 64 to 69, the circuit block X shownin any of FIGS. 1, 5, 6, and 8 to 63 described earlier may be applied.

PWM Control when Multiple Channels are Driven

FIG. 70 is a block diagram showing an electronic device comprising theload driving device (light-emitting diode driving device).

The electronic device shown in FIG. 70 has a microcomputer X10, alight-emitting diode driving device X20, a step-up/down circuit X30, anda light emitting part x40.

The microcomputer X10 is a means to collectively control the operationof the electronic device such as sending a luminance control command tothe light-emitting driving device X20.

The light-emitting diode driving device X20 is a semiconductorintegrated circuit device (LED driver IC), comprising a serial interfaceunit X21, a DC/DC converter unit X22, and a drive current control unitX23, which are integrated.

The serial interface unit X21 is a means of receiving the luminancecontrol command inputted from the microcomputer X10, and conveying thisto the drive current control unit X23. In FIG. 70, although aconfiguration of receiving the luminance control command (data signalDATA, a clock signal CLK, and a latch signal LAT indicative of a lightemitted diode to be turned on, or on duty and drive current valuethereof) by way of a three-line serial bus (I2C bus or the like) wasexemplified, the configuration is not limited to this, and a two-lineserial bus or a parallel bus may also be used.

The DC/DC converter unit X22 is a means for stabilizing an input voltageVin to generate a desired constant voltage Vreg.

The drive current control unit 23 is a means of generating drivecurrents (drive currents (I1 to In) to be supplied to each oflight-emitting diode rows LED1 to LEDn of n channels (n≥2) forming alight emitting part x40) of the light emitting part x40 according to theluminance control command inputted from the microcomputer X10 andcontrolling PWM [Pulse Width Modulation] thereof. Such PWM controlenables arbitrary adjustment of light-emitting luminance (and thuslight-emitting luminance of the light emitting part x40) oflight-emitting diode rows LED1 to LEDn, by variably controlling apparentcurrent values (average current values) of drive currents I1 to In. Thedrive current control unit X23 can be considered a circuit blockcorresponding to the first constant-current source I10 and the secondconstant-current source I20 in the second example (see FIG. 5), thesixth to eighth examples (see FIGS. 10 to 12), the twenty-seventh totwenty-ninth examples (see FIGS. 1 to 33), mentioned above. Theoperation of the drive current control unit X23 will be described indetail later.

The step-up/down circuit X30 is a means for boosting or lowering aconstant voltage Vreg generated by the DC/DC converter X22 to generate adesired drive voltage Vout and supplying it to the light emitting partx40 (anode ends of the light-emitting diode rows LED1 to LEDn). Thestep-up/down circuit X30 can be considered a circuit block correspondingto the power supply circuit 100 in the first to fifty-ninth examplesmentioned above. That is to say, the step-up/down circuit X30 isconfigured to perform output feedback control of a drive voltage Vout sothat among the first detection voltage Vdet1 to the nth detectionvoltage Vdetn each voltage level of which varies depending on amagnitude of each forward dropping voltage of the light-emitting dioderows LED1 to LEDn, a detection voltage at the lowest pressure levelcorresponds to a predetermined reference voltage. In addition, as suchoutput feedback control is similar to the first example to thefifty-ninth example mentioned above, a redundant description will beomitted. Although FIG. 70 exemplifies a configuration in which thestep-up/down circuit X30 is connected to the external of thelight-emitting diode driving device X20, the configuration is notlimited to this, and similar to the first examples to the fifty-ninthexamples mentioned above, the step-up/down circuit X30 may be built-inin the light-emitting diode driving device X20.

The light emitting part x40 comprises light-emitting diodes LED1 to LEDnof n channels connected to an anode as a common end in parallel, and isused as a backlight for illuminating a liquid crystal display televisionor a liquid crystal monitor for car navigation, for example. Inaddition, the number of serial columns of the light-emitting diode rowsLED1 to LEDn is not necessarily more than one, and a singlelight-emitting diode may be provided for each channel.

PWM control of drive currents I1 to In by the drive current control unitX23 will be described in detail hereinafter with reference to FIG. 71.

FIG. 71 is a waveform chart showing one example of PWM control. Theupper and lower stages of the figure, respectively, illustrate how PWMcontrol was performed conventionally and how PWM control is performed,with cycles T corresponding to each other.

In addition, the symbol PWM given at the left end of the figure shows alogical condition of the PWM signal, and the symbols I1 to I4 denotecurrent waveforms of the drive currents to be supplied to each of the4-channel light-emitting diode rows LED1 to LED4. In addition, thesymbol IDRV shows a current waveform of a total drive currents IDRV(total current of the drive currents I1 to I4) to be supplied from thedrive current control unit X23 to the light emitting part x40. Inaddition, the symbol T in the figure denotes a cycle of PWM control, andthe symbols Ton, Ton′ denote the on period of PWM control. In addition,the symbol i in the figure denotes a current value of the drive currentsI1 to I14.

As shown in the figure, for the light-emitting diode rows LED1 to LED4of all 4 channels, the light-emitting diode driving device X20 isconfigured to supply the drive currents I1 to I4 (a current value I forany channel) by shifting each on period Ton′ so that each of thechannels will not turn on simultaneously. More specifically, thelight-emitting diode driving device X20 is configured to turn on thelight-emitting diode row of one channel, and then turn on thelight-emitting diode row of a next channel after the on period Ton′ haselapsed.

With such configuration, unlike the conventional PWM control in whichthe drive currents I1 to I4 were supplied at the same timing to thelight-emitting diode rows LED1 to LED4 of all the four channels, a peakvalue of the drive currents IDRV can be reduced to each current value iof the drive currents I1 to I4, by preventing the drive currents IDRV(=4×i) for the four channels from flowing at a time during the on periodof PWM control. In other words, with the above configuration, as thetiming of heat generation of the light-emitting diode driving device X20is uniformly distributed, the heat generation efficiency thereof willrise. Thus, it becomes possible to reduce allowable dissipation of apackage and achieve a small footprint or cost reduction.

Since the human visual system perceives magnitude of luminance accordingto total energy amount to be given in unit time, sensible luminance willnot change considerably if my PWM control method is adopted.

In addition, frequency (1/T) of PWM control may be set to a number offrames of a displayed image (e.g., 30 [fps]) or frequency of acommercial AC power supply (50/60 [Hz]), and any frequency which doesnot match these multiples. By performing such frequency setting, itbecomes possible to prevent flickering in display images or illuminatinglight due to flashing of the lighting system 40.

When on duty of the conventional PWM control is α(α≥0) and when on dutyof the PWM control is β(β≥0), the on period Ton of PWM control of theconventional PWM control and the on period Ton′ of PWM control areexpressed in expressions (1) and (2), respectively:Ton=α×T  (1)Ton′=β×T  (2)

Hence, when the number of channels in the light-emitting diode row is n,a blank period Tded of the conventional PWM control and a blank periodTded′ of PWM control can be expressed in expressions (3) and (4),respectively:Tded=T−Ton=(1−α)×T  (3)Tded′=T−Ton′×n=(1−β×n)×T  (4)

Now the blank periods Tded, Tded′ should be Tded≥0, Tded′≥0, a range ofon duty α, β that can be set is expressed by expressions (5) and (6):0≤α≤1  (5)0≤β≤1/n  (6)

As can be seen from the expressions (5) and (6) above, in PWM control,there arises a restriction on an upper limit value (1/n) of the on dutyβ, depending on the number of channels n in the light-emitting dioderows. For example, when the light-emitting diode rows LED1 to LED4 ofthe four channels are driven, the on duty β cannot be set to more than0.25 (=¼).

Consequently, to obtain the light-emitting luminance (light-emittingluminance comparable to 0.25<α≤1 in the conventional PWM control) higherthan this, current values of the drive current should be increased fromi to i′ after the on duty β has reached a predetermined upper limitvalue (0.25).

Since total charges (then total drive currents) to be consumed in acycle T to obtain identical light-emitting efficiency is equal in PWMcontrol (channel distributed control) as well as in the conventional PWMcontrol (channel synchronization control), expression (7) is true:n×i×Ton/T=n×i′×Ton′/Tn×i×α=n×i′×β  (7)

From expression (7) above, a necessary current value i′ can becalculated from expression (8):i′=(α/β×I  (8)

For example, when the light-emitting diode rows LED1 to LED4 of the fourchannels are driven and the upper limit value of the on duty β in PWMcontrol is 0.25, it can be seen that to obtain the light-emittingluminance corresponding to the on duty α=0.5 of the conventional PWMcontrol, the current value i′ of the drive current may be increased to2×i.

Thus, the light-emitting diode driving device X20 is such configuredthat it performs PWM control only for the drive currents I1 to I4,unless the on duty β of the drive currents I1 to I4 has reached theupper limit value (0.25), and if the on duty β of the drive currents I1to I4 has reached the upper limit value (0.25), it performs not only PWMcontrol of the drive currents I1 to I4 but also current value control ofthe drive currents I1 to I4. Such configuration makes it possible tovariably control the light-emitting luminance widely in a setting rangesimilar to the conventional range, while reducing a peak value of thedrive current IDRV flowing through the drive current control unit X23.

In addition, in FIG. 70, although the description was given by taking asan example a configuration in which the light-emitting diode rows LED1to LED4 of the four channels are to be driven, the configuration is notlimited to this, and the number of channels of the light-emitting dioderows may be increased or decreased, as appropriate.

In addition, in FIG. 70, although the description was given by taking asan example a configuration in which for the light-emitting diode rowsLED1 to LED4 of the all four channels, the drive currents I1 to I4 aresupplied by shifting each on period so that each of the channels willnot turn on simultaneously, the configuration is not limited to this. Aslong as the configuration is such that it supplies the drive currents byshifting each on period so that the light-emitting diode rows in atleast one channel are prevented from turning on simultaneously with thelight-emitting diode rows of the remaining channels, it becomes possibleto reduce the peak value of the drive current IDRV flowing through thedrive current control unit X23 lower than the conventional peak value.

In addition, in FIG. 70, although the description was given by taking asan example a configuration in which immediately after light-emittingdiode rows of one channel are turned off, light-emitting diode rows ofother channels are turned on, the configuration is not limited to this.The configuration may be such that drive currents are supplied andcontrolled so that there is a predetermined simultaneous off perioduntil light-emitting diode rows of other channels are turned on afterlight-emitting diode rows of one channel are turned off. With suchconfiguration, it becomes possible to prevent a transient temperaturerise which occurs after a previous channel turns off from overlapping atemperature rise which accompanies turn-on of a next channel, and thusto improve the efficiency of heat dissipation thereof.

Also, in FIG. 70, although the description was given by taking as anexample a configuration in which current value control of drive currentsstarts after on duty of the drive currents has reached a predeterminedupper limit value, the configuration is not limited to this and may besuch that the current value control of the drive currents is performedeven before the on duty reaches the upper limit value.

In addition, in FIG. 70, although the description was given by taking asan example a configuration in which the microcomputer X10 inputs aluminance control command (data signal DATA, clock signal CLK, latchsignal LAT) to the light-emitting diode driving device X20, theconfiguration is not limited to this illustration, and may be such thata PWM signal of each channel is individually inputted from themicrocomputer X10.

In the above, although the description was given regarding preferredexamples, it is apparent to those skilled in the art that our devices,apparatus and methods can make modifications in various manners, andthat various examples which are different from the configurationsspecifically described above can be made. Hence, the appended claims areintended to include every modification in a technical scope withoutdeviating from the intent or technical perspective of this disclosure.

The invention claimed is:
 1. A light emitting load driving devicecomprising: a first constant current source structured to be seriallyconnected to a first light emitting load group; a second constantcurrent source structured to be serially connected to a second lightemitting load group; a first load connection terminal structured to beconnected to the first light emitting load group; a second loadconnection terminal structured to be connected to the second lightemitting load group; and a control circuit structured to be supplied afirst voltage applied to the first load connection terminal, a secondvoltage applied to the second load connection terminal, and a referencevoltage applied to the control circuit, wherein the control circuit isstructured to select a minimum voltage between the first voltage and thesecond voltage, and the control circuit is structured to equalize theminimum voltage and the reference voltage.
 2. The light emitting loaddriving device according to claim 1, wherein one end of the firstconstant current source is structured to be connected to groundrespectively.
 3. The light emitting load driving device according toclaim 2, wherein the first light emitting load group includes aplurality of light emitting diodes connected serially.
 4. The lightemitting load driving device according to claim 3, wherein the controlcircuit is structured to output a drive signal to a voltage supplyportion to be connected to the control circuit, the voltage supplyportion provides a voltage to the first light emitting load group andthe second light emitting load group.
 5. The light emitting load drivingdevice according to claim 4, wherein the first constant current sourceis structured to be controlled a volume of a current flowing thereto. 6.The light emitting load driving device according to claim 4, wherein thefirst constant current source includes a current mirror circuit.
 7. Thelight emitting load driving device according to claim 4, wherein a PWMsignal is provided from an output of the control circuit.
 8. The lightemitting load driving device according to claim 4, wherein the voltagesupply portion is structured to include a rectifying diode to rectify acurrent supplied thereto.
 9. The light emitting load driving deviceaccording to claim 4, wherein the voltage supply portion is structuredto include a smoothing capacitor to smooth a current supplied thereto.10. The light emitting load driving device according to claim 4, whereinthe control circuit is structured to control the voltage supply portionto generate the output voltage greater than or equal to 12 V.
 11. Thelight emitting load driving device according to claim 4, wherein thevoltage supply portion is structured to include an inductor connectedwith the rectifying diode.
 12. The light emitting load driving deviceaccording to claim 4, wherein the voltage supply portion is structuredto include a switching transistor to receive a signal provided from thecontrol circuit.